Molten metal filtration is the process used to obtain high quality metals suitable for casting. By removing undesirable impurities from the molten metal, the filtration process improves the products of the casting operation. Both mechanical and physical properties of casting products are enhanced by filtration.
In general, molten metal raw feed contains undesired impurities from sources such as particles of refractory from the lining of a vessel that contains the molten metal, alumina as a byproduct of deoxidization and reoxidation, fragments of slag or other insoluble impurities. Upon solidification of the cast product, these impurities adversely affect product properties such as surface finish, ease of drawing and forming, ease of welding, and strength. Therefore, a primary objective of the foundry industry is to remove impurities from molten metal raw feed by filtration. Filtration of the molten metal occurs prior to the casting operation and after the melting operation.
The melting operation involves melting metal so that it may be used in the casting operation. The metal is melted in a furnace wherein the constituent components are added in the form of unmelted scrap and/or refined virgin metal, deoxidizing agents in various forms (solid and gaseous or a combination of both) and alloying elements. Gases and low density solids tend to migrate to the surface of the melt where they either effervesce or float in combination with partially and completely solidified oxides commonly known as slag and dross. The higher density impurities in the melt tend to remain in the liquid phase of the metal, or melt, as the fluid flow convection currents are generated within that melt by the heat applied by the furnace.
During the melting operation, the furnace functions as a holding vessel for the metal while it is being melted. The furnace may also be used to refine the metal depending on what type of metal is being processed. Metal is refined when gases as well as low density metals migrate to the surface. The molten metal is transferred to another vessel, such as a ladle, to be transported to the molding operation. An alternative method would be to provide a direct flow path from the furnace to the casting operation. In both instances, prior to the casting/molding operations, the molten metal is routinely filtered.
The filtering system requires an efficient process to prevent solidification of the metal. Moreover, the filter medium must be suitable to withstand high melting temperatures and chemical reactions. Furthermore, the filter component must maintain its structural integrity. Lastly, the filter medium must be capable of either entrapping or preventing the flow of impure solids, liquids, and semi-liquids, all of which are non-metallic or intermetallic, either by chemically reacting with such impurities and/or by mechanically preventing the flow of such impurities through the filter medium, while still permitting and facilitating the flow of the molten metal through the filter.
Ceramic foam filters are commonly used in filtration operations by foundries. Ceramic filters reduce the number of castings that must be scrapped due to the presence of impurities and therefore improve casting cleanliness. Generally, ceramic filters are incorporated into the casting operation upstream of the mold cavity. The ceramic filters remove dross, slag and other impurities found in molten metal prior to the casting operation. The most common type of ceramic filters comprise hard-fired cellular ceramic structures and rigid reticulated ceramic foam. The ceramic filters, due to their mass, often chill the first molten metal that reaches the filter. Therefore, they require coarse openings to ensure reliable passage of the molten metal. Cellular extruded ceramic filters therefore rely on the formation of a filter cake on the upstream side to remove smaller inclusions that would tend to pass through the openings of the filter. Thus, ceramic foam filters are dependent on the formation of the filter cake to remove smaller inclusions. Due to the massive size and filtering problems that are inherent with the ceramic filters, fabric filters present a new opportunity for the casting operation.
At the present time, there is no commercially available filtration system which allows a continual replacement of a filter component which utilizes a fabric filter medium. A filter component having a fabric filter medium that is compact and effective during filtration would be desirable. It would be even more desirable to utilize such a filter in an automated, continuous filtration system.